Stefano,

Here's how to create and use categories and aliases.

CATEGORIES:

Each user can create their own categories.  To do this login to your account and go to your contacts.  Click on the hamburger menu in the bottom left corner and click "New Category".  A modal will open where you name your category.  To apply this to a contact, edit the contact and locate the Categories card.  In there toggle on the category you want to assign to that contact.  You can also assign multiple categories to one contact if needed.  Once you have the categories assigned there are two different ways to send an email to the contacts.

While still in your contacts you can filter to only show the contacts in a specific category.  You would then click the Select button and either use Select All in the drop down or click on individual contact cards to select only specific contacts.  Then click the drop down arrow on the Selected (#) button and you should see the "Send Email" option (see "send email from contact categories" screenshot).  When you click on that a new email compose window should open.

The second way to send to contacts in a category is to open a new email compose window first (see "select categories from an email" screenshot).  Click on the contact icon next to the "To" field (#1 in the screenshot) and a modal will open with all your contact lists.  In the modal open the List drop down menu to locate and select your category (#2).  Once you have the category selected you can either click "Select All" (#3) or select individual contacts from that category (#4).

ALIASES:

To create your group in an alias you'll need to have domain administrator access at the very least.  As the domain administrator go to Domain Settings > Accounts, and click on the "Aliases" tab at the top of the content pane.  In there click "New" and a modal will open.  Whatever you you name the alias, that will be the username for the alias email address.  So if you name your alias "SalesTeam" then the alias email address would be salesteam@domain.com.  The display name is just the same as when you use a display name for a contact (this is an optional field),  In the "Email Addresses" box you will list the email addresses of everyone you want included in the alias.  These can be any email address (yahoo, gmail, users on your domain).  There are other optional settings available for the alias you can select if you need them.  Once you've completed the set up of the alilas just click "Save" and you can start emailing to the alias address just as you would any other email address.